Most nitride-based single crystal semiconductor substrates such as GaN, used as a base when a semiconductor device is manufactured, are c-plane ({0001} plane) nitride thin films and are obtained by growing a nitride on a c-plane ({0001} plane) of a sapphire board through Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE) or Hydride Vapor Phase Epitaxy (HVPE).
A c-plane nitride-based single crystal layer formed in this manner has polarity since gallium and nitrogen, for example, are repeatedly deposited in the direction of c crystal axis. In the case of a c-plane GaN/AlGaN/InGaN heterostructure, for example, a strong electric field generated by spontaneous polarization or piezoelectric polarization tilts the electronic band structure in the heterostructure so as to reduce a carrier recombination rate, resulting in a reduction in quantum efficiency.
Specifically, polarization discontinuity is present in the c-crystal axis growth direction to generate sheet charges fixed to the surface or interface, and the resultant internal electric field separates electron and hole wavefunctions from each other to move an emission wavelength to a long wavelength and when an electric field is applied, the emission wavelength is moved to a short wavelength. This makes it difficult to develop devices for long-wavelengths.
On the other hand, a-plane ({11-20} plane) and m-plane ({1-100} plane) nitride-based crystals can overcome the above-described problem of the c-plane nitride-based single crystal, that is, the problem that quantum efficiency is decreased due to an internal electric field caused by polarization because the a-plane and m-plane nitride-based crystals have non-polar characteristics. The a-plane nitride-based crystals do not generate band bending since they have no polarization field, and stark effect is not observed in a structure having an AlGaN/GaN/InGaN quantum well formed on a non-polar crystal plane. Accordingly, an a-plane non-polar nitride-based heterostructure has a possibility that it can be used for high-efficiency light-emitting elements of an ultra-visible ray region and High Electron Mobility Transistor (HEMT).
Furthermore, an a-plane nitride-based layer can be doped in a concentration higher than that of a c-plane nitride-based single crystal layer because activation energy of the a-plane is 118 meV while that of the c-plane is 170 meV much higher than that of the a-plane. In addition, doping efficiency in GaN remarkably decreases as the content of Al in the GaN increases, in general, and thus heavy doping occurs in the a-plane compared to the c-plane.
The non-polar plane nitride-based single crystal layer cannot be manufactured and commercialized as a substrate although it has many advantages over the c-plane crystal layer because it is difficult to obtain a smooth surface and the non-polar plane nitride-based single crystal film has a large amount of internal defects compared to the c-plane crystal layer.
Specifically, an a-plane nitride-based single crystal layer is formed on an r-plane ({1-102} plane) sapphire single crystal base. In this case, a nitride layer having a surface shape in which ridges made of {1010} planes are extended in <0001> direction rather than a flat surface is formed, and strong compressive stress is applied in <1-100> direction of the nitride due to large anisotropy depending on a crystallographic orientation of an in-plane thermal expansion coefficient in addition to anisotropy of a lattice constant.
When this a-plane nitride-based single crystal is grown into a thick film or thin film, a film having no coalescence of ridges is formed to result in generation of a large amount of defects in the film. Poor surface shapes and defects make it difficult to manufacture devices and they are present on the surface of a substrate so as to deteriorate the performance of a final thin film device.